"It took me nine years to write this book."
With writing a book, it's always interesting to know when someone started and finished. I think this is partly because we become different people, over time, and it's interesting to know (very roughly) how many people wrote a book.
But these statements don't generate comparable data. Someone who wrote a book in nine years hasn't necessarily worked on it longer than someone who wrote a book in one. They haven't necessarily even been stuck for longer on how to progress their books.
Besides, you know you really began it sooner than you're claiming. You probably began it before you were born. It probably began you.
So that's why I find it strange when people say, "I spent nine years writing this book."
This blog post took me three feet, nineteen degrees, a lumen, a light-year and a cow's grass to write, so you'd better listen up.